Technical Field
The present disclosure is related to wireless communications systems, methods, devices, and computer-implemented processes. More specifically, the disclosure relates to power scaling operations for user equipment (UE) having dual connectivity to at least two different cells or transmission reception points.
Description of the Related Art
In the Third Generation Partnership Project (3GPP) for Evolved UMTS Terrestrial Radio Access Network (E-UTRAN, also referred to as Long Term Evolution or LTE including LTE-Advanced), there has been interest in having dual connectivity user equipment (UE). With dual connectivity, a UE in a Radio Resource Control (RRC) Connected state can be configured to simultaneously utilize radio resources provided by independent E-UTRAN Node Bs (eNodeBs or eNBs), one characterized as a Master eNB (MeNB) and the other characterized as a Secondary eNB (SeNB), operating on different frequency carriers.
In addition, multiple serving cells can be configured for the UE associated with the MeNB and/or SeNB. A master cell group (MCG) is a group of serving cells associated with the MeNB and a secondary cell group (SCG) is a group of the serving cells associated with the SeNB. The physical layer and the medium access control (MAC) layer of the MeNB and SeNB operate independently in a dual connectivity connection of the UE. The backhaul between the MeNB and SeNB is assumed to be non-ideal, which means that coordinated scheduling decisions between the MeNB and SeNB may not be feasible due to timing considerations.
One solution for achieving dual connectivity proposed by Nokia in meeting document R1-140560 of the 3GPP TSG RAN WG1 Meeting #76 is to is to coordinate random access channel (RACH) resources to prevent parallel physical RACHs (PRACHs) from being transmitted by a given UE in a time-overlapped fashion. In a synchronized network where coordination among the MeNB and SeNB cannot be assumed, the preambles of the MCG and SCG are alternated after the maximum transmit power is achieved, which may require that two MAC entities and the UE interact with each other to determine whether or not both preambles could be transmitted within the total transmission power budget. In addition, the two pending PRACH preambles may only partially overlap in the time domain (for instance, for only a portion of a symbol). The MAC layer typically operates on the granularity of subframes, which may not accommodate determining partial overlap in the time domain. The meeting document R1-140560 also suggests that the UE can transmit the preamble whose transmission was started earlier and drop the preamble associated with the later transmission.
3GPP TS 36.213 v11.5.0, section 5.1.1.1, the contents of which are incorporated herein by reference in its entirety, describes power control in a shared physical uplink channel. The UE is able to simultaneously transmit a PRACH on one component carrier, and one type of physical uplink control channel (PUCCH) on the other component carrier but is unable to transmit two PRACHs simultaneously. The types of PUCCHs can include PUCCH and physical uplink shared channel (PUSCH), PUSCH with uplink control information (UCI), PUSCH without UCI, and. Based on the transmission options provided in Release 11, the UE performs transmission power scaling if a total transmission power is going to be exceeded reducing a scheduled transmit power on the least prioritized channels until the total transmit power falls within the maximum allowable transmit power. The physical channels are prioritized in the following order (from highest to lowest): PRACH, PUCCH, PUSCH with UCI, PUSCH without UCI, SRS.